Soybean consumption has been associated with reduced risk for hormone dependent cancers in many epidemiological studies. Our previous studies have shown that consumption of soymilk containing significant quantities of the phytoestrogens, daidzein and genistein for one month effectively reduces levels of endogenous sex steroid hormones in men and women. These steroids are known to regulate breast and prostate cell proliferation and our results may explain the protective effects of soya diets against breast and prostate cancer development. The three aims of this study are to determine the chemopreventive component(s) of soya, the biochemical mechanism(s) by which soya mediates these endocrine effects and the time courses, dose-responses and the menstrual cycle phase specificity of soya consumption associated with these measurable endocrine changes. Adverse reproductive effect has been observed in few species of animals, but not others, and have been associated with the duration and quantity of phytoestrogen consumption, and are determined by inter-species difference in phytoestrogen metabolism. We have observed considerable interindividual, gender, and menstrual cycle phase-specific variation in the metabolism of phytoestrogens in humans. Based on animal data, such individual variability may influence both the potentially oncoprotective and adverse effects of soy. Attempts to prevent hormone-dependent cancers in humans by soya phytoestrogens will require, at least initially, close monitoring of the internal dose (e.g. plasma levels) in each individual. To better understand these interrelationships, we will determine the levels of isoflavones, sex steroids, and gonadotropins in the subjects before, during and after varying doses of soya phytoestrogen consumption. These and other ongoing studies will determine the optimal amounts of soya ingestion to favorably influence risk for developing hormone-dependent cancers.